In the serial joinder of tubular sections into long strings, it is often required to assure that there is both adequate mechanical strength and a leak-free seal in each joinder region. A significant example of this situation is found in the handling of pipe and casing strings under the demanding conditions encountered in present day oil well drilling. It is now common practice to drill to depths of more than 20,000 feet, which in turn requires that the successive couplings be capable of withstanding not only significant pressure differentials, but also extremely high forces along the longitudinal axis. The couplings must also be leak free under the conditions to be encountered, and couplings which do not respond correctly when made up must be replaced before the operation recommences.
Drill pipe and casings are of course fabricated with these practical requirements in mind, and different grades and weights of tubing may be used at different regions along the length of a string, with the tubing diameter usually, but not necessarily, remaining the same. Under presently used standards, each section of tubing has a male thread of specified length, taper and pitch at each end, to be received within a mating sleeve or collar having complementary female threads. The collar is sometimes referred to as the "box" or coupling and the pipe end is then referred to as the "pin". The nominal depth of insertion of the pin within the box can roughly be defined by reference to a "last scratch" mark that theoretically is the terminus of the thread on the pin. Manufacturing tolerances for the tubular sections and the collars are set by the American Petroleum Institute (API), which also has established the tolerances for the threads and the position of the last scratch (typically plus or minus two pitches). Pipe and casings are sufficiently expensive at the present time to make it economically impractical to place stricter tolerances than those presently observed with existing grades.
In making up a pipe string, a drilling crew must operate at maximum reasonable speed, sometimes being required constantly to exceed a certain minimum rate in order to maintain drilling mixture in the well bore. Individual pipe lengths are taken from a supply, usually with collars attached, and the pin end of the next succeeding section is threaded by a power tongs into the upstanding collar of the next previously attached section. An early technique was to rotate the pin a specific number of turns after a "handtight plane" engagement, while also checking the placement of the last scratch. This was an indirect method of trying to ascertain that a leak free coupling existed, and became clearly inadequate as drill string lengths and system operating pressures increased. Diameters, thread pitches, tapers and profiles of mating sections may be at opposite ends of the allowable tolerance ranges. Also, the amount of wear of the pin and box, the presence of foreign matter, the existence of damaged threads and other imperfections can greatly affect both the position of the handtight plane and quality of the coupling resulting after a number of turns are taken.
Other indirect measurement techniques, often based on measurement of torque, have long been attempted and in some cases used. None has proven to be satisfactory or practical for modern conditions, however, because both more precision and more positive assurance of a leak free seal are demanded. Workers in various arts have long used circumferential belt devices to attempt to measure the expansion of a cylindrical member. Engineering studies have also shown the resultant effect on a tubular collar of an internal force, such as an advancing threaded member.
As far as is known, only one attempt has been made to provide an essentially direct measurement of the expansion of a tubular collar for the purpose of assembling successive sections of tubular goods with leak free joinders. This approach is disclosed in U.S. Pat. No. 3,314,156 to Van Burkleo, who proposes the application of a bipartite flat band member that encircles a collar at some not precisely defined region between the ends of the collar, and is tightened to a starting condition. The relative position between a ferrite core on one part of the bipartite band and a coil on the other part of the band varies as the collar is deformed circumferentially when the mating threaded members are tightened together. Van Burkleo proposes usage of this variable reactance device to control the output frequency of a variable frequency oscillator which in turn control an indicating meter, either directly or by means of a remote transmission link. While the Van Burkleo device represents a creditable attempt to bypass indirect measurement techniques, it has never been commercially used although developed by a large industrial organization.
In the light of the present invention, the reasons for its impracticality are now understood, and derive from complexities that were not confronted by Van Burkleo. Van Burkleo proposed the use of an essentially flat strap and did not specify the circumference at which it was to be placed. Eccentricities in the members and foreign matter could thus affect the readings, which (it is now known) are widely variable where there is a substantial differential in the wall thicknesses of the threaded sections as they wedge together. Conversion of the true circumferential deformation to an electrical signal that is linear and repeatable is a considerable problem in both mechanical and electrical terms. The displacement may be very small even though the forces involved are large. Moreover, both long term and short term thermal effects can introduce major errors. For example, rapid tightening of the pin in the collar heats the collar significantly, while a less rapid tightening results in a lower temperature rise. Because temperature changes introduce high force variations, they can introduce substantial errors in taking readings of bearing pressure. Other temperature differences, both relatively stable and changeable, may also exist between the tubular sections and the sensor and affect the readings. Past systems, such as the Van Burkleo system, which did not recognize or take account of these factors, could not provide a reasonable expectation of providing useful output readings.
Since the Van Burkleo work, therefore, numerous attempts have been made by his organization and others to provide useful readings by less direct methods. In some systems, exemplified by U.S. Pat. Nos. 3,368,396, 3,606,664 and 3,745,820, torque measurement is combined with a measurement of the number of turns used in making the coupling. These techniques are not known to have been used in practice, at least partly because of the unreliability of the readings, but also because they slow down the drilling operations. It should be noted that measurement of applied forces in threaded couplings has been used in other contexts, as exemplified by U.S. Pat. Nos. 2,998,585, 3,541,844 and 3,872,719. In these systems a member is disposed within a load bearing structure to provide a locus for a strain gauge measurement of forces applied along the longitudinal axis, but this is neither directly applicable to nor economically useful in the handling of tubular products in a well drilling context. A number of laboratory examinations of stress conditions in cylindrical structures have been undertaken, as evidenced by an article by A. B. Potvin et al entitled "Stress Concentration in Tubular Joints", published in the Society of Petroleum Engineers Journal, pp. 287-299, Aug. 1977, and an article entitled "On The Use of Foil Strain Gauges to Measure Internal Strains of Thick Walled Cylinders" by R. V. Milligan, published in the ISA Transactions, Vol. 15, No. 1, pp. 95-99 (1976). The strain gauge techniques mentioned in these articles require careful preparation of the surface, alignment of the strain gauge and other precise adjustments, and even then merely provide a localized indication of the stress in the immediate region about the transducer.
It remains highly desirable, therefore, to provide a reliable, field usable, system for directly measuring the physical properties of a threaded joint to assure that it is both structurally sound and leak free, which system is independent of the variables and operative problems that can be encountered in practical applications, and functions in such manner as not to delay field operations.